a) Field of the Invention
The present invention relates to a charge transfer device which receives a plurality of charge in parallel to output the charge in series, a method of driving a charge transfer device, a charge-coupled device (CCD) image sensor including the charge transfer device, and a CCD image pickup system including the CCD image sensor.
b) Description of the Related Art
For example, a charge transfer device is produced on a semiconductor substrate as follows. A band-shaped n-channel is formed in a surface of the semiconductor substrate and a plurality of electrodes are arranged in parallel with each other to be adjacent to each other with an electrically insulating layer or simply an insulating layer between the n-type channel and the electrodes. In the configuration, the respective electrodes are disposed to intersect the n-type channel in a plan view. Such charge transfer devices including an n-type channel are mainly classified into three types as below.
In a charge transfer device of first type, the n-type channel has a substantially constant n-type impurity concentration and the insulating layer on the n-channel has also substantially constant thickness.
In the charge transfer device of this type, when voltages are applied to the respective electrodes, the potential well regions below the electrodes change such that a potential well region is formed below the electrode applied with a voltage at a relatively higher level and a potential barrier region is formed below the electrode applied with a voltage at a relatively lower level. When potential barrier regions are formed on the upstream and downstream sides of a potential well region, electric charge can be confined in the potential well region.
By appropriately controlling the voltages applied to the respective electrodes, the potential well region interposed between the two potential barrier regions can be sequentially moved in a desired direction. That is, the electric charge can be transferred in the desired direction.
In this specification, the movement of the charge transferred by the charge transfer device is regarded as a flow, and hence a relative position of each constituent member will be referred to, for example, as “at an upstream position of a unit A” or “at a downstream position of a unit B” according to necessity.
In a charge transfer device of second type, a region (to be referred to as an n+-type impurity doped region herebelow) having a relatively higher n-type impurity concentration and a region (to be referred to as an n-type impurity doped region herebelow) having a relatively lower n-type impurity concentration are alternately formed in the n-type channel.
In the CCD of this type, one electrode is disposed over the n+-type impurity doped region and one electrode is disposed over the n-type impurity doped region with an electrically insulating layer between the electrodes and the n-type channel. An electrode disposed over one n-type impurity doped region is commonly connected to an electrode disposed over an n+-type impurity doped region at an immediately or just downstream position of the n-type impurity doped region. In this situation, one electrode may be formed on the n-type impurity doped region and the n+-type impurity doped region just downstream thereof to cover these regions.
Each n+-type impurity doped region serves as a potential well region for the associated n-type impurity doped region in any case. Movement of charge is restricted by the potential barrier region. The electric charge can be transferred in the predetermined direction.
The operation will be described more specifically. Assume that an n+-type impurity doped region is “n+-type impurity doped region A”, an n-type impurity doped region at a just downstream position of the n+-type impurity doped region A is “n-type impurity doped region B”, and an n+-type impurity doped region at a just downstream position of the n-type impurity doped region B is “n+-type impurity doped region C”. Electrodes are respectively disposed over these regions B and C to be commonly connected to each other.
In a state in which charge is distributed to the n+-type impurity doped region A (potential well region), when a relatively higher voltage is applied to the electrodes respectively of the n-type impurity doped region B and the n+-type impurity doped region C, the n-type impurity doped region B does not serve as a potential barrier region for the n+-type impurity doped region A. The n+-type impurity doped region C is a potential well region for the n-type impurity doped region B in any situation. Therefore, the charge distributed to the n+-type impurity doped region A moves via the n-type impurity doped region B to the n+-type impurity doped region C.
The n-type impurity doped region B functions as a potential barrier region for the n+-type impurity doped region C. Even when the amplitude of the voltage of the electrodes respectively of the n-type impurity doped region B and the n+-type impurity doped region C is restored to the original value, the charge does not return from the n+-type impurity doped region C to the n+-type impurity doped region A at all.
In a charge transfer device of third type, the n-type channel has a substantially constant n-type impurity concentration. The insulating layer on the n-channel alternately includes a region (thick region) having relatively larger thickness and a region (thin region) having relatively smaller thickness.
Ordinarily, one electrode is disposed on each of the thick and thin regions. One electrode on a thin region is connected to one electrode on a thick region at just downstream position of the thin region. These electrodes are commonly connected to each other. In this case, one electrode may be formed on the thin region and the thick region just downstream thereof to cover these regions.
In the CCD of this type even when a fixed voltage is applied to the respective electrode, if the n-type channel is a buried type channel, a potential barrier region is formed below the thin region and a potential well region is formed below the thick region. The electric charge can be transferred in the predetermined direction.
A CCD image sensor is a representative electronic unit using the charge transfer device. CCD image sensors are mainly classified into two kinds, namely, CCD linear (line) image sensors and CCD area image sensors.
A CCD area image sensor ordinarily includes two kinds of charge transfer devices, that is, a vertical charge transfer device called “VCCD” and a horizontal charge transfer device called “HCCD”.
In a CCD area image sensor of inter-line type, a large number of photoelectric converter elements or photoelectric converters are arranged in a matrix-shape including a plurality of rows and a plurality of columns. A VCCD is disposed for each column of photoelectric converters. In many CCD area image sensors, all the VCCD is electrically connected to one HCCD. A CCD area image sensor including a plurality of horizontal charge transfer devices has also been known.
The VCCD is generally composed of a charge transfer device of a type in which the n-type channel has a substantially constant n-type impurity concentration and the insulating layer on the n-type channel has substantially constant thickness. The VCCD is ordinarily driven by vertical driving signals of three phases or more. In each VCCD, one electrode and a portion of the n-channel region therebelow forms one vertical charge transfer stage. About two to about four vertical charge transfer stages are disposed for one photoelectric converter.
In the HCCD, for example, an n+-type impurity doped region and an n-type impurity doped region are alternately formed in the n-type channel. A pair of electrodes commonly connected each other is disposed on a pair of adjacent n+-type and n-type impurity doped regions. These adjacent regions and the commonly connected electrodes form one horizontal charge transfer stage. Two horizontal charge transfer stages are disposed for one VCCD. The HCCD is ordinarily driven by two-phase horizontal driving signals.
A CCD system, for example, an electronic or digital still camera has been developed using a CCD area image sensor.
A digital still camera includes a small-sized monitor (display). A user of the camera can select a still picture recording mode to record a still picture and a monitor mode to display an image on the monitor. The monitor mode is used, for example, to determine an angle of view for a still picture.
The number of pixels of a still picture shot by a digital still camera has reached several millions these days and is about to exceed six millions. On the other hand, the number of pixels for a mobile picture displayed by the digital still camera in the monitor mode is about 100 thousand to about 400 thousand in general.
Therefore, in the monitor mode, charges are read only from part of the photoelectric converter rows to be stored into the vertical charge transfer devices. A thin-out scanning is conducted to thin out one half or more of the photoelectric converter rows. Or, an operation to mix charges in the VCCD (vertical addition) is performed. That is, in each photoelectric converter column, charges stored in two or more adjacent photoelectric converters are mixed in a VCCD (vertical addition). The addition of charges increases an amount of signal (charge) treated as one pixel in the signal processing. The shooting sensitivity advantageously increases in proportion to the increase of the amount of signal (charge) of the resultant signal. A relatively brighter picture or image can be reproduced.
In a CCD image pickup system for color pictures, a color filter array is used to obtain full color information. The color filter array includes a plurality of color filters repetitiously arranged in a predetermined pattern. One color filter corresponds to one photoelectric converter.
Image data in which the vertical pixels are thinned out can be obtained by conducting either one or both of the thin-out scanning and the mixing (vertical addition) of charges in the VCCD.
However, an HCCD capable of thinning out charges simultaneously (concurrently) received from the vertical charge transfer devices electrically connected to the HCCD has not been proposed. Similarly, an HCCD capable of conducting the mixing (horizontal addition) of charges has not been proposed.